1. Field of the Invention
The invention relates generally to separations and catalytic chemical reactions conducted in the gas or vapour phase. Some particular applications include gas already compressed and preheated oxygen for combustion, natural gas purification and compression, hydrogen isotope separation, ammonia or methanol synthesis, and steam reforming of methane.
2. Prior Art
Gas separation by pressure swing adsorption is achieved by cyclic flow of a gas mixture over an adsorbent bed which more readily adsorbs a fraction of the mixture. The term "swings" refers to periodic variations of pressure. The total pressure, ie, sum of partial pressures of the gases, is elevated during intervals of flow in a first direction and is reduced during alternating intervals of flow in the reverse direction. According to the well known parametric pumping separation principle, the less readily adsorbed fraction tends to migrate in the first direction over complete cycles while the more readily adsorbed fraction tends to migrate in the reverse direction.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds with directional valving to control the flow of compressed feed gas over each bed in alternating sequence, while the other bed is purged at low pressure by the reverse flow of a portion of the product gas. This conventional process makes inefficient use of mechanical energy, because the compression energy of the feed gas is largely dissipated over the valves. Another common name for pressure swing adsorption gas separation is "heatless adsorption", which seems to deny the possibility of a beneficial effect by thermal coupling to heat sources or heat sinks to improve pressure swing adsorption apparatus as disclosed in the present invention.
Conventional gas separation processes (including pressure swing adsorption, cryogenic distillation and membrane permeation) are powered by mechanical energy. The energy intensiveness of air separation is a major obstacle to widen use of enriched oxygen in high temperature processes such as solid fuel gasification. An efficient air separation process powered by waste heat would address this need.
Some secondary and adverse thermal effects do arise in operation of conventional pressure swing adsorption gas separation apparatus, particularly those using large adsorption beds with poor heat exchange to ambient. The adverse effects include cyclic release and take-up of the latent heat of adsorption which causes a temperature swing of the adsorbent bed which acts in opposition to the pressure swing. Adsorption is increased by higher pressure and/or reduced temperature, and conversely is decreased by lower pressure and/or higher temperatures.
Examples of single bed pressure swing adsorption devices are found in U.S. Pat. No. 3,121,625 (Broughton), U.S. Pat. No. 3,164,454 (Wilson), U.S. Pat. No. 3,236,028 (Rutan), U.S. Pat. No. 4,169,715 (Eriksson), U.S. Pat. No. 4,194,892 (Jones et al), U.S. Pat. No. 4,207,084 (Gardner), and U.S. Pat. No. 4,354,859 (Keller et al). Most of the above references use mechanical volume displacement means at one or both ends of the adsorbent bed to generate cyclic flow and pressure variations in the bed. The Keller patent uses mechanical volume displacement means at both ends of the adsorbent bed, with a specified range of phase angles between the two volume displacement means of unequal displacement. None of these references contemplates the integral coupling of a pressure swing adsorption process with a regenerative thermal power or heat pump cycle as in the present invention.
A temperature swing adsorption separation process known as "recuperative parametric pumping" was described by Wilhelm (R. H. Wilhelm, A. W. Rice and A. R. Bendelius, Ind. Eng. Chem. Fundamentals 5, 141, (1966) with applications to liquid phase separation. A fluid mixture was made to flow cyclically in forward and reverse directions in an adsorbent bed with an imposed thermal gradient between hot and cold ends. The heat capacity of the fluid mixture was large with respect to the heat capacity of the adsorbent material, in order to obtain thermal cycling of each adsorbent pellet.
The present invention can be considered as subjecting a flow of a gas mixture to a Stirling cycle or other thermodynamic regenerative cycle which has been changed to operate as an open cycle to perform thermally coupled pressure swing adsorption separations. This contrasts with a conventional Stirling cycle machine which has a closed working volume filled with a gaseous working fluid. The working volume includes hot and cold spaces, whose volume is varied by mechanical volume displacement means reciprocating out of phase. A thermal regenerator is in the flow path connecting the hot and cold spaces. In a conventional Stirling machine heat capacity of the working fluid is desirably very low with respect to the heat capacity of the regenerator materials so that the temperature swing at each point of the regenerator will be minimal. This temperature swing of the regenerator results in non-isothermal conditions in the regenerator and is an unavoidable source of inefficiency in conventional Stirling type machine.
The out of phase reciprocation of the volume displacement means causes cyclic flow of the working fluid through the regenerator. Cyclic pressure variations are associated with alternative heating or cooling of gas flowing respectively into the hot or cold spaces, and with variations of total working volume. If volume changes in the hot space have a leading phase with respect to volume changes in the cold space, the Stirling machine is an engine. Conversely, the Stirling machine is a heat pump or refrigerator when volume changes in the hot space lag volume changes in the cold space. Related machines include thermocompressors, Vuilleumier thermally powered refrigeration machines, and Gifford-McMahon pressure powered refrigeration machines.
In Stirling type machines operated as cryogenic refrigerators, performance deteriorates at very low temperatures where the heat capacity of regenerator materials ceases to be very large relative to the heat capacity of the helium gas working fluid. As disclosed in U.S. Pat. No. 3,262,277 (Nesbitt), a solid adsorbent material can improve performance of thermal regenerators operating below 20 degrees K., as the effective heat capacity is increased by the presence of adsorbed helium in the regenerator material. Under the steady state operating conditions envisaged by Nesbitt, the adsorbed helium phase is essentially static, contrary to the requirements of a gas separation or purification process. Significant rates of cyclic adsorption and desorption would entail corresponding release and take-up of a latent heat of adsorption, degrading regenerator thermal performance in a Stirling type refrigerator. Successful operation of the Nesbitt patent depends on suppression of cyclic pressure swing adsorption and desorption through substantial cancellation by opposing temperature swing effects over the adsorbent in the regenerator, and probably also through capillary condensation of liquid helium in adsorbent pores which causes known pressure hysteresis effects inhibiting desorption from pores filled with liquid adsorbate. High speed operation may also tend to suppress cyclic adsorption and desorption.
Use of rapidly dissociating gases such as nitrogen tetroxide as working fluid in Brayton and Stirling closed cycle engines is proposed in U.S. Pat. No. 3,370,420 (Johnson). The dissociation/recombination reaction increases gas volume in the hot space and thus improves engine work ratio. Use of a catalyst in the regenerator of a Stirling engine using a dissociated gas working fluid has been proposed in U.S. Pat. No. 3,871,179 (Bland) with the object of obtaining enhanced reaction rates in closed cycle Stirling engines with high work ratio. In these inventions, the forward reaction is exactly cancelled by the reverse reaction over each cycle because reaction products are trapped in the engine spaces. As there is no means to drive the reaction off equilibrium, these inventions cannot be applied to chemical synthesis processes. Therefore it has not hitherto been possible to apply the closed Stirling cycle to recover heat from exothermic chemical synthesis reactions, or to supply heat for endothermic reactions, while using the reacting gases as the Stirling machine working fluid.
Fundamental problems in chemical process industry include the removal of reaction product species and exothermic reaction heat from catalyst beds. High temperatures promote good reaction rates, but shift the equilibrium of an exothermic reaction toward lower conversion. These problems and their conventional solutions are exemplified by the important chemical process of ammonia synthesis, which proceeds by the exothermic reaction: EQU 3H.sub.2 +N.sub.2 .revreaction.2NH.sub.3
This reaction takes place over a promoted iron catalyst at a typical pressure of 200 atmospheres and a typical temperature of 750 degrees K. The hydrogen and nitrogen feed gases are stringently purified (apart from minor amounts of "inert" gases such as argon and methane, which are non-reactive in the ammonia synthesis loop, and are compressed to the high working pressure. In order to remove the product and thus drive the synthesis reaction over the catalyst bed off equilibrium, the gas mixture of reagents and produced ammonia is recirculated between the hot catalyst bed and a cool ammonia separator/condenser. This recirculation requires a recycle compressor and a recuperative heat exchanger. To prevent excessive catalyst heating from the exothermic reaction, temperature control is achieved either by energy inefficient quenching by injection of cool feed gas, or by heat exchange to an external waste heat recovery power cycle. A Brayton cycle gas turbine heat recovery approach for ammonia synthesis is disclosed in U.S. Pat. No. 4,224,299 (Barber et al) and U.S. Pat. No. 4,273,743 (Barber et al). Unless the synthesis loop operates at very high pressure, a refrigeration plant is needed to condense liquid ammonia at the cool end of the synthesis loop. Means are provided for purging accumulated inerts from the loop, and valuable hydrogen from the purge gas.
Considerable research attention has been devoted to improving productivity or selectivity of catalytic chemical reactors through cyclic operation forced by periodic variation of feed composition or temperature cycling. For example, it was found by (A. K. Jain, Ph.D. Thesis, University of Waterloo Ontario) that forced feed composition cycling at periods of several minutes improved the productivity of the ammonia synthesis reaction. However operation of the ethylene hydrogenation reaction over a nickel catalyst with pulsating pressure and flow was tested by Baiker et al (A. Baiker and W. Richarz, Chem. Ing. Tech 48, 1203, (1976)), who found that catalyst productivity was reduced relative to steady state operation. While it has been shown that in many cases cyclic operation can improve reaction productivity or selectivity under laboratory conditions, there remains a need for full scale reactors capable of beneficially exploiting a wide range of periodic phenomena which may be based on cyclic composition, temperature, flow or pressure variations.
Chromatographic effects have been found to enable some catalytic reactions to be driven beyond normal equilibrium constraints, when the reverse reaction is suppressed by opposite separation of products species as pulses of a feed reactant migrate through a catalyst bed in the presence of a continuously flowing carrier gas, which may be a second reactant. Chromatographic reactors are disclosed in U.S. Pat. No. 2,976,132 (Dinwiddie and Morgan) and in Canadian Pat. No. 631,882 (Magee). It was found by Unger and Rinker (B. Unger and R. Rinker, Ind. Eng. Chem., Fundam; 15, 226 (1976)) that ammonia synthesis could be conducted to high conversions beyond equilibrium at relatively low pressure by pulsing nitrogen, with hydrogen as the carrier, through a packed bed of catalyst mixed with adsorbent. These chromatographic reactors have severe limitations, including low catalyst productivity because of the interval between feed pulses, and the mixing of dilute product species in the carrier gas.
Closed cycle Stirling machines have been developed strictly as devices for conversion between thermal and mechanical forms of energy. The complex internal regime in which pressure, temperature and velocity vary continuously with time and location in Stirling machines has not previously been found useful for gas separations or chemical synthesis between components of the working fluid as taught in the present invention.